Intra-annular conformers

Figure 2.8. Attempts to threading by covalent bond formation. Only the extra-annular conformer 12 was obtained. The intra-annular conformer (11) was not observed.

A huge group of macrocycles which contain 2,6-disubstituted phenols are the calixarenes [51]-[53]. Their conformation has been investigated intensively (Gutsche, 1989, 1991). In most conformations, however, the phenolic rings are oriented almost vertically in relation to the plane of the macrocydic ring. Therefore the OH functions are not oriented in an intra-annular fashion. Nevertheless the pK values of calixarenes [51] differ from those of other comparable phenols. The reason for this is the... 

Figure 12. Conformation of ferrichrome A. The cyclohexapeptide ring has only one intra-annular hydrogen bond, 4 - 1 type II (Zalkin et al., 1966).

In order to interpret the low-temperature spectra, COSY determinations were conducted at -50°C. COSY of IX confirmed that the (singlet) signals at 7.14, 7.08, 5.86 and 5.83 ppm (see Figure 4d) are due to the intra-annular protons on C41-44. A conformer which would result in the intra-annular protons being observed as four singlets is a partial cone conformer which is analogous to those... 

The -50°C COSY spectrum of X is considerably more complex than that of IX. At least eighteen pairs of doublets and up to four singlets can be discerned in the methylene proton region (Figure 7). At least six signals (singlets) due to the intra-annular protons can also be discerned, albeit with difficulty. As with IX a partial-cone conformer could account for eight pairs of doublets a 1,2-altemate (unsymmetrical) conformer could account for an additional six pairs of doublets and a 1,2-altemate (symmetrical) conformer could account for an additional four... 

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